A fuel cell is an electric power generation system for directly converting chemical reaction energy of hydrogen and oxygen included in a hydrocarbon-based material such as methanol, ethanol, natural gas, or the like into electrical energy.
Fuel cells are classified into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte membrane fuel cells, and the like according to the type of electrolytes. The basic principles of these fuel cells are identical to each other, but they are different from each other in terms of the type of fuels, operating temperature, catalyst, electrolyte, and the like.
Polymer electrolyte membrane fuel cells (PEMFCs) have remarkably high output performance, low operating temperature, rapid start and response characteristics, and a wide application range compared to other fuel cells.
In a fuel cell system, a substantial power-generating fuel cell stack has a laminate structure of unit cells each including a membrane-electrode assembly (MEA) and a separator provided with a gas passage. The membrane-electrode assembly is configured such that anode and cathode electrodes are attached to each other with a polymer electrolyte placed therebetween. That is, the membrane-electrode assembly has a laminate structure of a polymer electrolyte membrane, two electrodes, a catalyst layer, and a gas diffusion layer.
When hydrogen is supplied to an anode, it is oxidized into hydrogen ions and electrons by an electrochemical oxidation reaction. The hydrogen ions are transferred to a cathode through the polymer electrolyte membrane, and the electrons are transferred to the cathode through an external circuit. The hydrogen ions, which are transferred to the cathode, cause an electrochemical reduction reaction together with the oxygen supplied to the cathode to produce heat and water, and electrical energy is generated by the movement of the electrons.
After a polymer electrolyte membrane fuel cell stack is operated for a set amount of time, its performance is deteriorated due to the degradation of a platinum-carbon (Pt—C) electrode and a polymer electrolyte membrane constituting a membrane-electrode assembly (MEA). Platinum particles adjacent to a high-voltage cathode are eluted or lost by oxidation, or are dissociated by the corrosion of carbon supporting platinum, thus reducing the electrochemical surface area (ECSA). Further, carbon monoxide (several parts per million (ppm)) included in fuel is chemically adsorbed on a platinum catalyst, thus decreasing a hydrogen oxidation reaction (HOR) rate. Additionally, a local temperature increase occurring during the operation of a high-power vehicle shrinks pores of a membrane or rearranges SO3− terminal groups, thus decreasing ion conductivity.
Further, it is known that the performance of the fuel cell stack is deteriorated during the long-term storage thereof as well as the degradation of the MEA occurring at the time of operating the high-power vehicle. That is, in the case where the fuel cell stack is stored for a long time, when air charged in the anode and cathode (air/air (anode/cathode)) is left for a long period of time, the voltage of each of anode and cathode becomes about 1.0 V. In this case, an oxide film is formed on the surface of platinum exposed to oxygen and droplets at a high voltage of about 1.0 V according to the following Chemical Formula to prevent reactive oxygen from being adsorbed on the surface of the platinum during the operation of a fuel cell, thereby decreasing a reduction reaction rate.
[Chemical Formula]Pt+H2O->Pt—OH+H++e−
The performance deterioration of a fuel cell stack occurring during the storage thereof mostly returns to the original performance thereof through a platinum reduction procedure attributable to reversible deterioration. However, when a vehicle is operated in a state in which the oxide formed during the storage of the fuel cell stack remains on the surface of platinum, irreversible deterioration, such as the elution of platinum from oxide, is accelerated, thus deteriorating durability of the fuel cell stack for fuel cell vehicles.
It is to be understood that the foregoing description is provided to merely aid the understanding of the present disclosure, and does not mean that the present disclosure falls under the purview of the related art which was already known to those skilled in the art.